Trapping of optical excitation in Nd-doped PrF3

The dynamics of trapping of optical excitation in a disordered system of ions in a solid is studied using time-resolved fluorescence measurements. In the system Pr${\mathrm{F}}_3$ doped with Nd, the randomly distributed Nd ions act as sinks for the Pr excitation. The transfer to the Nd traps can occur in conjunction with transfer among the Pr ions. In this system the Pr-Pr transfer is strongly temperature dependent while the Pr-Nd transfer is temperature independent at low temperatures. By changing the temperature we can span the range from no Pr-Pr transfer to very (infinitely) fast Pr-Pr transfer in one single system. We have identified these limits. In addition we have placed boundaries on the applicability of the diffusion and hopping models in describing trapping in the intermediate temperature regime where the Pr-Pr and Pr-Nd transfer rates are comparable.